This invention relates to space fed antenna array systems, and more particularly, to wide band space fed antenna systems used in chirp or frequency hopping radars.
In space fed antenna systems, such as the antenna system shown in FIG. 1 of U.S. Pat. No. 3,305,867, issued to A. R. Miccioli and D. H. Archer on Feb. 21, 1967, entitled "Antenna Array System" and assigned to the assignee of this invention, radio frequency (R.F.) radiation to be collimated and directed is radiated by a transmitter horn which illuminates an electronic lens. The electronic lens is composed of a plurality of phase shifters responsive to a programmer. Different portions of the R.F. radiation from the transmitting horn are received by the plurality of phase shifters wherein predetermined corresponding phase shifts are imparted to the different portions of such R.F. radiation to radiate a beam of R.F. energy with a uniform phase front in a predetermined direction. However, the direction of the beam of R.F. energy radiated by the electronic lens is frequency dependent. For example, in a chirp or frequency hopping radar system employing an antenna system as described above, if a target is imaged in the far field with the beam of R.F. energy centered on that target at a predetermined frequency and that frequency is changed, the beam will squint off the target. The amount of squint error, .DELTA. sin .theta..sub.0, is given by the equation: ##EQU1## wherein .DELTA.f is the frequency difference of the radiated R.F. energy from the selected operating frequency f and .theta..sub.0 is the scan angle of the radiated R.F. radiation off boresight of the antenna system. Should the frequency difference (.DELTA.f) be sufficiently large, the target could be lost completely. This squinting is a result of each of the phase shifters of the plurality of phase shifters in the electronic lens giving a constant phase shift, regardless of frequency, relative to other ones of the plurality of phase shifters. This squint error has the effect of moving the focal point parallel to the plane of the electronic lens for a fixed far field beam position. In addition, the effective focal length of the electronic lens varies with frequency, causing collimation errors. An obvious solution would be to physically move the feed horn (or the electronic lens) to reposition the focal point on the feed horn. This physical movement is slow and requires devices of considerable mechanical complexity, leading to an unwieldy, expensive and unreliable system. Another solution is to operate the radar over small frequency bands over which the squinting error is acceptable and resetting the phase shifters if another frequency band is used. However, if wide band operation is desired, as in a chirp radar, the number of such small frequency bands becomes large, thereby requiring a large number of resets of the phase shifters during a chirp interval. Further, if a large number of phase shifters is used in the electronic lens, e.g. ten thousand, a large amount of time would be spent resetting the phase shifters, thereby reducing the effectiveness of the radar. Additionally, the power handling capability of such antenna system is limited to the power handling capability of the transmitter horn.